Endotoxin or lipopolysaccharide (LPS), released from the cell wall of bacteria, plays a central role in a broad spectrum of human disease. The pathogenic importance of LPS in gram-negative sepsis is well established. Intravenous LPS induces all of the clinical features of gram-negative sepsis, including fever, shock, leukopenia followed by leukocytosis, and disseminated intravascular coagulation (Favorite et al., 1942). Higher concentrations of circulating levels of endotoxin have been associated with manifestations of systemic inflammatory response syndrome (Wang et al., 1995) and the development of acute respiratory distress syndrome following sepsis (Brigham et al., 1986). Inhaled endotoxin can induce airflow obstruction in naive or previously unexposed subjects (Michel et al., 1992) and is the most important occupational exposure associated with the development Schwartz et al., 1995a), and progression (Schwartz et al., 1995b), of airway disease among exposed workers. The concentration of endotoxin in the domestic setting appears to be associated with the clinical severity of asthma (Michel et al., 1996). Moreover, recent studies have shown that endotoxin is a contaminant of particulate matter in air pollution and may play a role in the pathophysiologic consequences of air pollution (Bonner et al., 1998). Thus, endotoxin is an important cause of morbidity and mortality.
The ability of the host to respond to endotoxin may play an important role in determining the severity of the physiologic and biologic response to this frequently encountered toxin. In mice, genetic differences in susceptibility to LPS have been established. LPS hyporesponsiveness arose spontaneously and was first identified in the C3H/HeJ strain. This strain had an LD50 for LPS at least 20 times that observed in A/HeJ mice (Sultzer et al., 1968). In addition to C3H/HeJ, two other mouse strains, C57BL/10ScCR (Coutinho et al., 1978) and its progenitor strain, C57BL/10ScN, (Vogel et al., 1979), are hyporesponsive to LPS.
Moreover, several reports suggest that humans may also respond differently to LPS. A patient with recurrent bacterial infections has been reported to be refractory to the in vivo and in vitro effects of LPS (Kuhns et al., 1997). Following challenge with intravenous LPS, this patient had no systemic increase in IL-6 or G-CSF and had a minimal rise in the concentration of leukocytes, TNF-α, and IL-8. Inter-individual differences have also been reported in the release and synthesis of cytokines by monocytes stimulated with LPS in vitro (Santamaria et al., 1989).
LPS is thought to cause much of its morbidity and mortality by activating kinases (DeFranco et al., 1998) that control the function of transcription factors (nuclear factor-κB and AP-1) and ultimately lead to production of pro-inflammatory cytokines and co-stimulatory molecules (Wright, 1999). Several lines of evidence suggest that the toll receptor (TLR) family, and specifically TLR4 and TLR2 regulate the interaction between LPS and intracellular kinases and may serve as a proximal target to interrupt LPS signaling (Wright, 1998; Medzhitov et al., 1997). Both TLR4 and TLR2 activate signaling through NF-κB and AP-1 in transfected human cell lines (Medzhitov et al., 1997; Yang et al., 1998), and TLR4 mediates LPS induced signal transduction (Chow et al., 1999). CD14, a glycosylphosphatidyl inositol-linked receptor that binds LPS (Poltorak et al., 1998a) enhances LPS induced TLR2 (Yanget al., 1998) and TLR4 (Chow et al., 1998) signaling, suggesting that the toll receptors interact with CD14 to initiate the cellular response to LPS. Studies in mice indicate that 1) the TLR4 gene maps to the critical region in LPS hyporesponsive mice (Poltorak et al., 1998), 2) mutations in the TLR4 gene (Poltorak et al., 1998; Qureshi et al., 1999) are found in mouse strains (C3H/HeJ and C57BL10/ScCr) that are defective in their response to LPS, and 3) disruption of the TLR4 gene results in a LPS hyporesponsive phenotype (Hoshino et al., 1999).
Thus, there is need to determine whether the human TLR4 gene is polymorphic, and whether any particular polymorphism is associated with disease, e.g., LPS hyporesponsiveness.